Electrolytic process

ABSTRACT

PROCESS FOR THE ELECTROLYSIS OF MOLTEN OXIDES, ESPECIALLY OF ALUMINA, IN WHICH A GAS PERMEABLE ANODE IS SEPARATED FROM THE MELT BEING ELECTROLYZED BY A LAYER IN CONTACT WITH THE ANODE AND THE MELT, OF AN OXYGEN-ION-CONDUCTING MATERIAL, FOR EXAMPLE ZIRCONIUM OXIDE STABILIZED WITH CALCIUM OXIDE OR OTHER OXIDES, WHICH IS RESISTANT TO THE MELT AT THE TEMPERATURE OF THE ELECTROLYSIS.

United States Patent US. Cl. 204-67 1 Claim ABSTRACT OF THE DISCLOSUREProcess for the electrolysis of molten oxides, especially of alumina, inwhich a gas permeable anode is separated from the melt beingelectrolyzed by a layer, in contact with the anode and the melt, of anoxygen-ion-conducting material, for example zirconium oxide stabilizedwith calcium oxide or other oxides, which is resistant to the melt atthe temperature of the electrolysis.

CROSS REFERENCE TO RELATED APPLICATION This application is a division ofmy copending application Ser. No. 638,249, filed May 15, 1967, now U.S.Pat. 3,562,135, patented Feb. 9, 1971.

BACKGROUND OF THE INVENTION The electrolysis of molten materials, forexample of alumina, is carried out today with carbon anodes. In the caseof alumina, the oxygen ions formed during the electrolysis react withthe carbon of the anode at the process temperatures of 900 to 1000 C.and form carbon dioxide, which is partly reduced to carbon monoxide bythe aluminium itself. Owing to the oxidation of the anode by the nascentoxygen, the carbon anode is consumed and, in fact, if only carbondioxide were to be formed, 334 kg. of carbon per ton of aluminiumproduced would be consumed. In practice, about 400 to 450 kg. of anodecarbon are consumed per ton of aluminium, which corresponds to about 8to 10% of the cost of crude aluminium. It has only recently beenpossible to reduce the consumption of anode carbon even to this figure,and with the present method of working using carbon anodes, reduction ofthe consumption of anode carbon below the theoreticall smallest amount,that is 334 kg. of carbon per ton of aluminium, is not possible.

It has now been found that it is possible to carry out the meltelectrolysis of oxides without using carbon anodes, but using electrodeswhich are oxygen-resistant Without necessarily being resistant to attackby the melt being electrolyzed, so that the oxygen can be obtained as avaluable by-product of the process. About 600 mm. of pure gaseous oxygenare formed per ton of aluminium; the value of the oxygen is about 3% ofthe cost of the crude aluminium. The oxygen which can be recovered incarrying out the process according to the invention into effect can beused for various oxidising processes, such as for example, steelproduction (by the oxygen blow method), the gasification of fuels (forproducing synthetic gas) and the preparation of reducing gases for ironreduction.

According to the invention I use an anode of any suitable conductingmaterial, and I separate this anode from the melt being electrolysed bya layer of material which is oxygen-ions-conducting but non-permeable toand resistant to the melt at the temperature of the electrolysis, sothat the oxygen ions diffuse through the layer and are then dischargedat the anode with the formation of oxygen gas. The anode itselfpreferably consists of an electronice conducting material which does notreact with oxygen or at least does not form with oxygen any compoundimpairing the conduction of electrons. Suitable materials includeheat-resistant alloys, platinum or other noble metals,electron-conducting oxides, such as for example, wiistite, certainmaterials with semi-conductor properties, and metals with a passivatedsurface. The thickness of the oxygen-ion-conducting layer may be verysmall so that the voltage drop across it is also small; this reduceslosses of energy during the electrolysis.

SUMMARY I have found that know stabilised forms of zirconium oxide arevery suitable as the material which separates the anode from the melt.By stabilised I mean zirconium oxide in which is incorporatedproportions of other oxides such as calcium oxide, magnesium oxide andyttrium oxide, which serve firstly to stabilise the cubic (fluorite)lattice of the zirconium oxide, and secondly to confer on it thenecessary oxygen-ion conductivity. By suitable choice of oxides andtheir proportions, a stabilised zirconium oxide can have a resistance aslow as 10 ohms-cm. at 1000 C. Other refractory oxides which havefluorite lattices can be used, such as for example, rare earthoxide-uranium oxide compositions, thorium oxide-uranium oxidecompositions and cerium oxide suitably stabilised with calcium oxide ormagnesium oxide. Substances which reduce the solubility of theoxygen-ion-conducting material may be added to the fused melt.

The invention will be described hereinafter with specific reference tothe electrolysis of alumina for the production of aluminium. In such anelectrolysis, the oxygen ions which are formed in accordance with theequation dilfuse through the oxygen-conductive layer and are dischargedat the anode in accordance with the equation i.e. the oxygen ionscombine to form oxygen gas and electrons are released in the process.These electrons are conducted away by the anode. Other oxides such asfor in stance, MgO, Na O, CaO, Fe O can also be electrolysed by theprocess according to the invention and similar equations can beformulated. In the electrolysis of alumina for example, cells accordingto the invention atford the following advantages, inter alia, incomparison with the present state of the art. I

(1) There is no consumption, or only a very low rate of consumption, ofanode material with the result that the rate of production of anodematerial can be substantially reduced.

-(2) The formation of carbon scum in the bath results in a loss ofoperating efficiency, and this formation will not occur if the anodesare of material other than carbon.

(3) There is improvement of the quality of the aluminium metal produced,since no impurities, such as iron, silicon or vanadium are introducedfrom the anodes material.

(4) There is less down-time of the cell, since the anodes do not have tobe replaced.

(5) There is a reduction of the consumption of fiuxing materials, sincethe cell can be sealed off more satisfactorily and this also gives animprovement in the shop atmosphere.

(6) Pure oxygen can be produced and collected as a byproduct.

(7) There is no re-oxidation of the liquid aluminium by carbon dioxideand thus, there is increased output from the cell and a reduction in theenergy required to produce a given weight of aluminium.

Cells according to the invention can readily be adapted for automationof operation with for example, continuous addition of alumina to thefused melt and maintenance of constant interelectrode gap or cellvoltage.

Cells according to the invention may be constructed in two Ways. In thefirst of these, the anode is coated with or is in contact with the layerof oxygen-ion-conducting material over at least that part of its surfacewhich is immersed in the melt; the anode must then be in such a physicalstate that oxygen gas can pass through it.

The anode may be solid, in which case it must be porous, perforated orreticulated.

If the anode is solid, the layer of oxygen-ion-conductin'g material maybe applied directly to it by pressing or casting with subsequent dryingand sintering or by plasma spraying. Alternatively a body of thematerial may be preformed quite separately and put in contact with theanode, if the latter is, for example, a metal network. As a furtherpossibility, a porous layer of platinum black may be applied to apreformed body of the material, and electrically connected to oneterminal of the current supply. This last proposal is found to be verysatisfactory, as platinum black is particularly suitable for thedischarge of oxygen ions and the formation and removal of oxygen gas.

What is claimed is:

1. The method of reducing molten oxide by electrolysis in anelectrolytic cell which comprises passing a direct electric currentbetween a cathode in contact with the melt and a gas permeable anoderesistant to the formation with oxygen of any compound impairing theconduction of electrons, said anode being separated from the moltenoxide of the melt by a layer of solid material in contact with the anodeand immersed in and in contact with the melt and which isoxygen-ion-conducting but non-permeable to and resistant to the materialof the melt at the temperature of the electrolysis, and said current asit passes through the cell causing diffusion of oxygen-ions through saidlayer of material with the formation of oxygen gas References CitedUNITED STATES PATENTS 6/1971 Stewart 20464 X 9/1968 Tasiri et a1.204--11 JOHN H. MACK, Primary Examiner N. A. KAPLAN, Assistant ExaminerU.S. Cl. X.R. 20464 R,

